Rubber products such as an electric wire, a rubber hose (also referred to as a rubber tube), a tire, a grommet or a vibration-proof rubber have been widely used as each member for which physical properties or characteristics such as mechanical characteristics, flexibility, elasticity, repellency, and permanent compressibility are required. As a rubber material from which these rubber products are formed, a wide range of rubber materials such as ethylene-propylene-diene rubber (EPDM), styrene-butylene rubber (SBR), nitrile-butylene rubber (NBR), and fluorine-containing rubber have been used. Moreover, a crosslinked polyethylene material has been widely used as a coating material or a member for various cables by taking advantage of heat resistance thereof.
These rubber materials and crosslinked polyethylene are produced into the rubber product as described below. More specifically, a crosslinking agent such as organic peroxide and a phenolic compound is previously blended into rubber, and the resultant blend is molded in a state in which these crosslinking agents do not sufficiently react therewith. Then, a crosslinked molded body having rubber elasticity and flexibility is obtained by heating the non-crosslinked molded body to cause crosslinking, and cooling the resultant material. For example, in a case where the electric wire is continuously produced, the rubber material or the like is molded at a low temperature of 120° C. or lower and in this state, for example, passed through a vulcanization pipe warmed by water vapor or the like to cause crosslinking, and the resultant material is further passed through a cooling pipe cooled by water or the like.
Thus, in a case where the rubber material or the crosslinked polyethylene as described above is used, upon molding these rubber materials or the like, it is required to mold the materials at a temperature at which the crosslinking agents cause no reaction, and then sufficiently heat the molded material at a temperature at which the crosslinking agents are decomposed to cause reaction, while keeping the molded state, to progress crosslinking, and to cool the resultant material. Therefore, a long period of time is required for production thereof.
Moreover, usually, the rubber material or the like should be molded at the temperature at which the crosslinking agents cause no reaction, which has posed a problem of difficulty in molding the material by a specific method such as injection molding.
As a method of solving these problems, proposals have been made on a method of dynamically crosslinking, by using organic peroxide through metal hydrate subjected to silane surface treatment, a vinyl aromatic thermoplastic elastomer composition prepared by using a thermoplastic elastomer, or a block copolymer described in Patent Literatures 1 to 3, or the like as a base resin, and adding a softener for non-aromatic rubber as a softener. However, while these thermoplastic elastomers have flexibility, these elastomers are melted at a high temperature, and therefore are unable to be used as the rubber product.
Incidentally, specific examples of a method of crosslinking a polyolefin-based resin such as polyethylene include an electron beam crosslinking method using an electron beam, and a silane crosslinking method.
However, in the electron beam crosslinking method, not only cost for facilities is significantly high, but also a thickness of the molded body which can be produced is restricted, and therefore such a method is unable to be applied for the various rubber products. On the other hand, the silane crosslinking method is a method of obtaining a crosslinked molded body, by a grafting reaction of a silane coupling agent onto a polymer in the presence of organic peroxides, to obtain a silane graft polymer, and then contacting the silane graft polymer with water in the presence of a silanol condensation catalyst. This silane crosslinking method requires no special facilities in many cases. Accordingly, among the above-described crosslinking methods, the silane crosslinking method has been particularly applied in a wide range of fields.
Usually, in a case where a filler is mixed with a resin, a Banbury mixer, a kneader mixer or a twin screw extruder is used. However, if the kneader or the Banbury mixer is used in a case where the resin containing the filler is crosslinked by the silane crosslinking method, a silane coupling agent is volatized before a silane grafting reaction because of high volatility. Therefore, it becomes difficult to prepare a silane master batch containing a silane graft polymer and the filler. Moreover, also in a case where the twin screw extruder is used, problems of difficulty in resin pressure control and easily causing foaming remain.
Therefore, in the case of preparing a silane master batch with a Banbury mixer or a kneader, consideration might be given to a method which includes adding a silane coupling agent to a master batch prepared by melt-mixing polyolefin and an inorganic filler such as a flame retardant or reinforcement material, and then subjecting the resultant to the silane coupling agent is reacted onto polyolefin so as to form a graft in a single-screw extruder. However, this method may cause poor appearance. Moreover, if an antidegradant is incorporated into the master batch, inhibition of the silane grafting reaction is caused, and desired heat resistance is unable to be obtained in several cases.
As another method, Patent Literature 4 describes a method in which an inorganic filler surface-treated with a silane coupling agent, a silane coupling agent, an organic peroxide, and a crosslinking catalyst are melt-kneaded with olefin-based resin using a kneader, and then the blend is molded using a single-screw extruder. However, according to the method described in Patent Literature 4, the olefin-based resin resins are crosslinked with each other during melt-kneading in a kneader, and the crosslink causes poor appearance. Further, a greater part of silane coupling agent other than the silane coupling agents with which the inorganic filler is surface-treated, is volatilized or the silane coupling agents are condensed with each other. For this reason, the desired heat resistance cannot be obtained and, in addition, poor appearance may be caused by condensation of the silane coupling agents.